Invited review at the conference "Quasars as
Standard Candles for Cosmology", La Serena, Chile, May 1998,
ed. G. Ferland, ASP Conference Series, in press.
For a postscript version of the article, click
For a postscript version of the article, click here.
Abstract. Key to our understanding of the workings of the most powerful energy sources in the universe, active galactic nuclei (AGN), are full, multi-wavelength observations of their emission. In this article the current observational status of AGN spectral energy distributions (SEDs) throughout the radio--ray spectral regions is reviewed along with our intepretation of them and discussion of key areas which remain insufficiently explored.
The past decade has seen great strides in a number of spectral regions including: X-ray, -ray, infrared (IR), mm; and has resulted in observed SEDs for a larger and more representative fraction of the AGN population. This has led to a, sometimes reluctant, concensus on the important energy generation mechanisms in the various wavelength bands. However there are key spectral regions which remain beyond our observational capabilities, the EUV in particular, and regions where the full potential of current instrumentation has not yet been achieved, far-infrared and mm.
The inter-relation between the emission in the various spectral bands is discussed along with currently popular scenarios to explain both these relations and the SEDs in general. This is followed by a discussion of the role of dust and related emission line studies as well as the newly recognised class of NLSy1 galaxies. The article concludes with the suggestion, based on the results in a number of widely different studies discussed here, that orientation may play an important role in determing the observed properties of AGN with the pole-on view being provided by NLSy1 galaxies.
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